This invention relates to grinding methods and particularly relates to grinding of ceramic materials to ceramic powders. The invention especially relates to autogenous attrition grinding of such ceramic materials.
There has been a need for high temperature ceramic materials in powdered form wherein the particle size of the powder is very small, e.g., submicron. Such ceramic powders, for example, in submicron size, i.e., average particle size of less than one micron, are especially required for sintering operations wherein the powders are sintered into high temperature, high hardness ceramic articles. In the prior art, especially for materials having high hardness, e.g., in excess of nine on the Mohs scale, it was exceedingly difficult to obtain powders having particle sizes as small as desired. In order to obtain such powders, exceedingly long grinding times, often as long as days, were required. Furthermore, due to the hardness of the material, it was difficult to grind such materials without contamination, such as iron, resulting from the grinding media and container. It was proposed, for example, in U.S. Pat. No. 4,275,026, to grind ceramic materials such as titanium diboride in a mill having surfaces constructed of a non-contaminating material such as titanium diboride itself. When grinding media was used, it was usually a shaped media. The ground material had a broad particle size distribution with a large weight percent being in the larger particles. Surface areas indicate that the average particle size is usually not submicron even with long grinding times. An attrition mill is mentioned but there is no suggestion of high energy input in such a mill.
It has also been proposed, for example in U.S. Pat. No. 3,521,825, to actually introduce a second phase material in a milling process by including a milling media which provides the second phase material upon grinding in a milling jar. An attrition mill is not suggested for any purpose. This patent requires grinding media balls or pellets and involves slow milling processes. The milling time in the example in U.S. Pat. No. 3,521,825 is 72 hours.
Another method for avoiding contamination of product in such milling operations is by coating the walls of the container with an abrasion resistant material such as rubber or polyurethane which is satisfactory for slow grinding operations of the prior art.
It has further been proposed, for example in Bulletin 670 of the United States Department of the Interior, U.S. Government Printing Office No. 1981-332-076, entitled "Comminution By The Attrition Grinding Process" by Stanczyk et al, that ceramic materials can be ground using the material itself as the grinding media in a higher energy process. The process as disclosed in Stanczyk et al, however, has serious shortcomings. In particular, it is generally disclosed that a grinding media such as silica sand is desirable. Furthermore, the process and equipment disclosed is neither coated with an abrasion resistant material nor coated with the material being ground. In addition, the reference generally does not disclose grinding energy input which provides an agitator tip speed any greater than 7.22 meters per second. Such an energy input still is sometimes not as high as desirable to obtain rapid grinding of the material. Furthermore, especially in grinding devices which are coated with an abrasion resistant surface such as rubber or polyurethane, heat which is developed during the grinding process simply cannot be removed rapidly enough through the wall of the device to prevent steaming and build-up of pressure during a wet grinding operation at higher energy input. Such autogenous mills have not been suggested for use to reduce an oxygen sensitive feed material to a surface area of at least 5m.sup.2 /g nor to an average particle size below 1 micron. Additionally, it had been believed that such grinding resulted in rounded particles. Rounded particles of narrow size distribution are usually not considered desirable for sintering operations due to poor compaction properties.
Certain milled materials such as AlN, TiB.sub.2, Si.sub.3 N.sub.4 and sialons have been found to have characteristics unsuitable for sintering. The reasons for such unsuitability have not been entirely clear, but the presence of surface oxygen is believed by the inventors herein to be an important consideration which adversely affects sintering.
A paper by S. Prochazka dated August 1986 (General Electric Technical Information Series 86CRD158) discusses the milling of ceramic powders especially by attrition grinding. The paper discloses milling of various materials including mullite, alumina, silicon nitride, silicon carbide and boron carbide in various liquid media. There is, for example, a disclosure in Table 1 of the reference of silicon nitride milling in 2-propanol using zirconia media. There is no disclosure of milling silicon nitride to obtain a powder suitable for sintering and there is no suggestion that attrition milling of oxygen sensitive ceramics in an oxygen deficient environment would serve any useful purpose.
It is disclosed in copending patent application Ser. No. 722,272 that ceramic powders can be ground in a vibro-energy or vibratory mill, i.e., a vibrational mill having high frequency and special media. Frequency is often between 750 and 1800 cycles per minute. Such vibro-energy mill grinding, however, has serious disadvantages. In particular, high density packing of certain vibrationally ground ceramic materials is not obtained as easily as desired and vibratory grinding sometimes introduces impurities, especially boron and aluminum, which are undesirable for certain applications, especially electronics. Such impurities often result from sintering aids used in the manufacture of the grinding media. In addition, media of similar composition to the material being ground is difficult to manufacture and is expensive because submicron material must be blended with sintering aids, shaped and sintered. The use of sintering aids may introduce undesirable impurities as previously discussed.